VARIATION OF ZR/RB RATIOS IN THE CHINESE LOESS SEQUENCES DURING THE LAST 130KYR AND ITS IMPLICATION FOR CHANGES IN WINTER MONSOON STRENGTH

We have selected seven loess-paleosol sections covering the last 130kyr in the Loess Plateau of China for measuring concentrations of the trace elements Zr and Rb as well as the grain size. Variations of Zr/Rb ratio along the sequence in all of the sections display a generally similar pattern to that of the grain size which have been generally considered as a proxy of the East Asian winter monssn strength. This close relationship has been also verified by a spatial gradient distribution of the two parameters in the seven sections along the N-S transect across the Loess Plateau. Further analyzed data of Zr and Rb concentrations in five separated particle fractions show that the ratio of Zr/Rb increases as the grain size increases. This well explained a positive linear relation of the ratios and values of the mean grain size existed in all of the least weathered loess samples. Acid solution experiments indicated that both Zr and Rb are immobile elements during pedogenic processes. Therefore, the variations of Zr/Rb ratios in the sequences can be regarded as a better index for the winter monsoon strength than the mean grain size. Matching the Zr/Rb record with the mean grain size curve suggests that strength of the winter monsoon during the marine isotopic stage 3 is particular weak. This means the Asian monsoon could be driven by changes of the solar insolation on the precessional time scale.